The present invention relates to a method and apparatus for sensing an image using a solid state image sensor including a plurality of pixels, and more particularly, to a method and apparatus for correcting an image, suitable for use with an X-ray image sensing apparatus.
With recent advances in the technology of image sensing apparatus, it has become possible to realize a large-sized image sensing apparatus using a great number of photoelectric conversion elements, and capable of sensing a high-quality large-area image with a high resolution. Such recent advances in the technology also make it possible to acquire radiation image information such as an X-ray image for use in medical examination directly through a high-resolution and large-sized image sensing apparatus instead of using a conventional silver halide film. Such radiation image information may be converted into digital electronic information for further various processing.
FIG. 1 is a simplified block diagram illustrating an example of the basic structure of an X-ray image sensing apparatus using a solid state image sensor. In FIG. 1, reference numeral 1 denotes an X-ray generator, 2 denotes a fluorescent plate for converting an X-ray to visible light, 3 denotes an optical system, and 4 denotes a solid state image sensor.
The X-ray generated by the X-ray generator 1 passes through an object 15 to be examined, if there is such an object, and incident on the fluorescent plate 2. If there is no object, the X-ray is directly incident on the fluorescent plate 2. The fluorescent plate 2 serves as a wavelength converter for converting the X-ray to light with a wavelength sensible by the solid state image sensor 4.
The fluorescent light generated by the fluorescent plate 2 is focused via the optical system 3 onto a two-dimensional solid state image sensor 4 which senses image information using a CCD or a similar device comprising a plurality of photoelectric conversion elements.
Reference numeral 8 denotes a clock generator for generating a basic clock signal by which the two-dimensional solid state image sensor 4 and other circuit devices are driven. Reference numeral 9 denotes a control signal generator for generating various control signals on the basis of the clock signal. Reference numeral 5 denotes an analog-to-digital (A/D) converter for converting the signal output by the solid state image sensor 4 to a digital signal, which is supplied over a signal line 12. Reference numeral 10 denotes a device including a memory (dark output data memory) for storing the data representing the level (dark level) of the signal output by the solid state image sensor 4 when there is no input signal. In normal operation, a subtractor 6 subtracts the dark level from the signal output from the solid state image sensor 4, and outputs the resultant signal over a signal line 13. Reference numeral 11 denotes a device including a memory (shading memory) for storing image data obtained when there is no object such as a human body 15 to be examined. This data represents the shading distribution including the variation in the conversion efficiency from one photoelectric conversion element to another of the solid state image sensor 4. A divider 7 divides actual image data taken in a normal mode by the data stored in the memory 11 so as to make a correction in terms of the shading effect including the variation in the conversion efficiency of the photoelectric conversion elements. The corrected data is output over a signal line 14.
When an actual X-ray image is taken, the solid state image sensor is first driven under the condition that no X-ray is generated, and the obtained dark output signal is stored in the dark output data memory 10. Then an X-ray is generated under the condition that there is no object such as a human body to be examined, and an image signal is taken via the solid state image sensor 4. The image data is reduced by an amount corresponding to the dark output data, and the result, which includes the shading effect and the variation in the conversion efficiency of the photoelectric conversion elements, is stored in the shading memory 11.
If it is assumed that an X-ray is incident on the fluorescent plate with uniform energy across the fluorescent plate, and if the X-ray is converted by the fluorescent plate to visible light with uniform conversion efficiency, then it is expected that the solid state image sensor 4 will provide an equal output signal for all pixels. Under the above conditions, if there is a variation in the output signal when there is no object to be examined, the variation in the output signal can be considered to arise from the variation in the conversion efficiency of the photoelectric conversion elements. If this output signal is reduced by an amount corresponding to the output signal which is output from each photoelectric conversion element when there is no incident X-ray, then the result represents the net output of each photoelectric conversion element for the maximum incident energy of the X-ray.
From this net output signal, it is possible to obtain shading data, although the data usually includes the variation in the conversion efficiency of the photoelectric conversion elements.
In the next step to obtain image information, an image is taken under the condition that there is an object such as a human body to be examined. The subtractor 6 removes the dark output from the image signal, and furthermore the divider 7 corrects the image signal in terms of the shading effect and the variation in the conversion efficiency of the photoelectric conversion elements. The corrected signal is output over the signal line 14.
In the conventional technique described above, it is required to generate an X-ray to obtain correction information about the shading effect and the variation in the conversion efficiency of the photoelectric conversion elements. If the X-ray is generated to obtain correction information each time an image is taken, the life of an X-ray tube is wasted by the nonessential operation. Furthermore, such an operation for obtaining correction information whenever an image is taken results in a great increase in the operation time. Thus it is desirable that the operation of obtaining correction information about the shading effect and the variation in the sensitivity of the photoelectric conversion elements be performed at rather long time intervals, such as once every day.
In general, however, the location of the X-ray generator relative to the location of the image sensing apparatus is moved from time to time during a day for convenience of examinations. This can cause a substantial change in the shading condition.
FIG. 2 is an one-dimensional illustration of a change in the shading shape due to a change in the position of the X-ray generator. In FIG. 2, the horizontal axis represents the pixel positions, and the vertical axis represents the output of the photoelectric conversion elements. Data 31 represents the initial shading shape, and data 32 represents the shading shape obtained after the X-ray generator is moved from the initial location. As can be seen, variations occur over the entire shading shape.
In practice, the distribution of X-ray radiation intensity is not uniform but rather gradually varies such that the intensity becomes maximum near the central position, as represented by curves 31' and 32'. Furthermore, small variations in the photoelectric conversion efficiency from pixel to pixel are superimposed on the X-ray radiation distribution 31' or 32', and thus the overall distribution can be represented by an upward convex curve including small fluctuations as is the case in data 31 and 32 shown in FIG. 2.
That is, data 31 and 32 represent the overall shading characteristic at respective locations of the X-ray generator. In FIG. 2, line 33 represents the output obtained by performing the correction process described above. Although small variations due to the variations in the conversion efficiency from one photoelectric conversion element to another are well corrected on the basis of the correction data obtained for the respective locations of the photoelectric conversion elements, the difference between data 31 and 32 due to the shift of the X-ray intensity distribution results in an undesirable slope in the overall characteristic.
This can be understood by the fact that when there are two functions both having a similar shape which is smooth and upward convex, if the center of one function is shifted from the center of the other, and if one function is divided by the other, then the relative value inverts at a point where the two functions intersect each other.
As described above, the X-ray intensity distribution produces an undesirable effect in the corrected data. This effect can result in an artificial and unnatural modification in the image data. Because the image data taken through the solid state image sensor is used in all the following processes, it is desirable that the image data include no undesirable modification.